Reaction products of dialkylol thioureas and disubstituted phenylenediamines as antiozonants



United States Patent OTice REACTION PRODUCTS OF DIALKYLOL THIO- UREASAND DISUBSTITUTED PHENYLENEDL AMINES AS ANTIOZONANT S Harry E. Albert,Lafayette Hill, Pa., assiguor to The Firestone Tire & Rubber Company,Akron, Ohio, a corporation of Ohio No Drawing. Original application Jan.31, 1956, Ser. No. 562,596. Divided and this application Mar. 20, 1958,Ser. No. 722,662

6 Claims. (Cl. 260-453) This invention relates to reaction products ofdialkylol thioureas and N,N-dialkyl phenylenediamines as antiozonants inrubber.

The dialkylol thioureas may be prepared in any suitable manner as, forexample, by the aldehyde process of Pollak, F., Modern Plastics 16, No.10, 45, 74, 76 (1939). Various aldehydes can be used in preparing thedialkylol thioureas as, for example, formaldehyde, acetaldehyde,propionaldehyde, or butyraldehyde. The dialkylol thioureas have theformula HOCHR.NH.CS.NH.CHR.OH

in which each R can be hydrogen or an alkyl group of l to 3 carbonatoms.

The N,N'-dialkyl phenylenediamines can be ortho or para isomers and canhave nuclear substituents. The alkyl substituents can be the same ordilferent, and each can contain from 1 to 12 carbon atoms. Althoughthese disubstituted phenylenediamines are antiozonants in rubber, manymembers of the class are extremely toxic, and for this reason it isdiflicult to handle them in commercial operations. Others are non-toxicand very useful.

Oxygen and ozone both have a harmful effect on rubber, but the effect ofeach is difierent, and compounds which inhibit or prevent the harmfuleffect of one are not necessarily effective in stopping or retarding theharmful efiect of the other.

Crabtree and Kemp in an article in Industrial and Engineering Chemistry,vol. 38, starting at page 278 (1946) explain the difference in theaction of oxygen and ozone on rubber. The light-catalyzed oxidationwhich occurs during outdoor exposure forms a skin and crazed appearanceover the exposed surface of the rubber. Ozone, on the other hand, evenin -very low concentration, induces cracking in rubber, but only if therubber is stretched. (C. H. Leigh-Dugmore, Rubber Age and Synthetics,November and December 1952.) The cracks are perpendicular to thedirection of stretch. Such cracking can occur in the absence of light.Compounds which inhibit the effect of ozone on rubber are referred toherein as antiozonants.

Tires are stressed when inflated. While a tire is at rest it isstretched statically, and on a moving vehicle it is stretcheddynamically, i.e., it undergoes alternating stretching and relaxing.Some of the antiozonants are more effective in static tests and othersare more effective in dynamic tests. Antiozonants which are effectiveunder both conditions will be desired for tires, but for other rubberproducts an antiozonant which does not meet both tests may be used.

The inhibiting eifect of the antiozonants of this invention on rubberwas determined by treatment of unaged, cured stocks with air ofcontrolled ozone content in specially designed equipment. The test wasconducted with one-half inch dumbbell samples of approximately 100 gaugethickness. The special apparatus for testing with air of controlledozone content, and the method of a 2,956,98i Patented Oct. 18, 1960 2 7testing therein are described in the articles by Ford and Cooper,appearing in India Rubber World for September and October 1951, entitledA Study of the Factors Affecting the Weathering of Rubber-LikeMaterials-'I and II. The following test results refer to a test in whichthe ozone concentration was maintained at 60 parts per 100,000,000 partsof air. Two types of tests were conducted. In one type, called thedynamic test, the sample was repeatedly stretched between the limits of0 and 20 percent elongation at the rate of 108 cycles per minute. In theother type of test, the static test, the samples were stretched at 12.5percent elongation through out the test. On completion of each test thenumber and size of the cracks in each sample were compared visually withthe number and size of the cracks in a blank strip from the samemasterbatch which contained no antiozonant and which was cured andtested at the same time as the test sample. The number of cracks wasreported on an arbitrary scale as follows: none, very, very few (or v.v.few), very few, few, moderate, "moderate-to-numerous, and numerous. Thesize of the cracks was reported according to an arbitrary scale asfollows: very slight (or v. slight), slight, mod-- erate, severe, andvery severe.

In the reported test 2.0 parts by weight of the antiozonant andN,N-di-sec-butyl-p-phenylenediamine, respectively, were added to themasterbatch for each parts of rubber present. The latter stock was usedas a control.

The reported results include data on the tensile properties of the curedrubber stocks before and after aging 2 days in an oven at 212 F. Themodulus and tensile strength are given in pounds per square inch, andthe elongation is reported as percent of stretch at the break. Thesedata are included to show that the antiozonant has no substantialdeleterious effect upon the cure or upon the aging of the cured stock.

The antiozonants are effective in the vulcanizates of both natural andsynthetic rubbers, as, for example, those produced by polymerizing orcopolymerizing a conju: gated diene, as, for example, 1,3-butadiene (orhydro carbon homologue thereof) with an ethylenically unsaturatedmonomer as, for example, styrene, acrylonitrile, methacrylonitrile, oran ester of vinyl alcohol, as ester of acrylic or methacrylic acid,vinylpyridine, vinylcarbazole or other loW-molecular-weight vinylmonomer, etc.

The rubber can be vulcanized with sulfur or a sulfur donor, or with across-linking agent such as a dithiol, nitroor nitroso-compound, aquinone dioxime, etc; Known vulcanization accelerators are useful inspeeding up the vulcanization process and are operative herein,especially the relatively active accelerators including the thiazolesulfenamides, e.g., N-cyclohexyl-2-benzothiazolesulfenamide, thiazolinesulfenamides, thiocarbamyl sul-' fenamides, mercaptothiazoles,mercaptothiazolines, thiazolyl monoand di-sulfides, theN,N-disubstituted dithio-i carbamates, the thiuram sulfides, thexanthogen sulfides, and metallic salts of mercaptothiazoles ormercaptothiazolines or dithiocarbamic acids.

One or more accelerator activators are often used with any of theaccelerators mentioned, and such activators include the variousderivatives of guanidine known in the rubber art, amine salts ofinorganic and organic acids, various amines themselves, and alkalinesalts such as sodium acetate and the like, as well as other activatorsknown in the art. Additionally, two or more accelerators or acceleratorcombinations are sometimes desirable in a single rubber compound. Manyof the accelerators mentioned above are suitable in latex formulations,especially such common accelerators as piperidinium pent-amethylenedithiocarbamate, zinc butylxanthate, zinc ethylxanthate, zinc salt ofmercaptobenzothiazole, zinc 3 dimethyldithiooarbamate, and zincdibutyldithiocarbamate. Although vulcanization is usually accomplishedby heating a vulcanizable rubber composition at a temperature in therange of 240 to 400 F. for a time ranging from several hours to a fewseconds, vulcanization does take place at lower temperatures such asordinary room temperature. It is quite common to vulcanize a latex filmcontaining an ultra-accelerator by allowing the film to remain at roomtemperature for several hours or a few days.

The following example illustrates the preparation of the reactionproducts.

EXAMPLE 1 To 6.3 grams (0.08 mole) of thiourea in 50 ml. of water therewas added 12.5 grams of aqueous formaldehyde (36% by weight=0.l mole). Afew drops of acetic acid were added as the catalyst and the mixture wasstirred for two hours at 45 to 50 C. Then 33 grams (0.15 mole) ofN,N'-di-sec-butyl-p-phenylenediamine was added and the reaction mixturewas stirred two hours at room temperature. It was then heated for twohours at 95 C. Thirty-five grams of a dark brown viscous liquid wasobtained.

Other N,N-dialkyl phenylenediamines which can similarly be reactedinclude the following:

0- and p-Phenylenediamines N-methyl, N-sec-buty1 N-l-methylbutyl,N'-1-methylheptyl N-octyl, N'sec-bntyl N-undeeyl, N'-isopropylN,N'-diethyl N,N'-di-n-butyl N,N'-didodecyl N,N'-dimethyl N,N'-dihexylN,N-dinony1 N-methyl, N'-undecyl N-sec-butyl, N'-dodecyl N-isopropyl,N'-arny-l N,N'-diisopropyl N,N'-di( l-methylheptyl) N,N'-dil-methyloctyl) In carrying out the reaction one or both hydroxyl groupsof the dialkylol thiourea react with hydrogen of one or both of theamino groups of the N,N'-dialkyl phenylenediamine or with the nuclearhydrogen atoms. The product of Example 1 was prepared using a ratio of0.5 mole of dialkylolthiourea to 1 mole of N,N- dialkylphenylenediamine.A larger ratio of dialkylolthiourea may be employed, the theoreticalupper limit per mole of N,N'-dialkylphenylenediamine being n/2 moles ofdialkylolthiourea where n/ 2 is the number of reactive hydrogens(nuclear or on nitrogen) in the N,N'- dialkylphenylenediamine molecule.Actually it would not be desirable to use a mole ratio greater thanabout 2 for the use of a ratio which is too high would give too manydimethylenethiourea cross-links between theN,N'-dialkyl-p-phenylenediamine molecules.

Tests were made using a tread stock masterbatch having the followingformula:

Parts by weight 100 Sulfur 1.7 Black 45 Stearic acid 2.5 Zinc oxide 2.4Softener 6.6 Accelerator 1.2

The test strips were cured 80 minutes at 280 F.

A blank of the foregoing composition was tested simultaneously with acompound containing two parts of the reaction product prepared accordingto Example 1,

TABLE I Blank Test Control Masterbatch 159.4 159.4 159.4.

Reaction Product of Example 1 N, N di-sec-butyl-pphenylenediamine.Normal Tensile Properties:

300% Modulus Tensile strength Elongation After Aging (2 days at 212 F300% Modulus Elongation Ozone Effects (4 hrs., 60 pphrd,

slightno. Frequency v.v. fem, none. Dynamic:

Size Frequency moderate" numerous The foregoing shows that the producthas good antiozonant properties. It is more persistent than the N,N'-di-sec-butyl-p-phenylenediamine used as the control. This is a naturalconsequence of its having a greater molecular weight.

An important advantage of the reaction product of Example 1 is its lowertoxicity as indicated in the following table in which the sensitivityeffect is recorded. These results were determined by exposing 15 humansubjects in each case to patch tests using 5% of the test material incold cream. Two patch tests of 48 hours duration were used, the timebetween the two exposures being about 14 days. The results of the secondpatch test are indicative of the sensitization effect. The degree of theeffect is noted by using a scale of 1 to 4 plus signs.

Hence the reaction product of Example 1 is considerably less sensitizingthan the compound from which it is prepared.

The antiozonant of Example 1 was compared withN,N'-di-sec-butyl-p-phenylenediamine and with a blank (no antiozonant)from the standpoint of natural weathering at Akron, Ohio, using thetread stock formulation given previously. Dumbbell strips were exposedat an elongation of 12.5% in this test. At the end of 28 days the stockcontaining the antiozonant of Example 1 and the stock containing N,N di-sec -butyl p phenylenediamine showed no cracking while the stockcontaining no antiozonant showed severe cracking. After 2 monthsexposure, the tire stocks prtoected by antiozonants were essentiallyequal. Hence, these data again demonstrate the compound of Example 1 tobe an effective antiozonant.

The antiozonant will be used in any small amount as, for example, 0.1 to10 parts per parts of rubber. This application is a division of myeopending application Serial No. 562,596, filed January 31, 1956, nowabandoned.

The invention is more particularly defined in the claims which follow.

What I claim is:

1. Sulfur vulcanized rubber of the class consisting of natural andsynthetic rubbers which are from the class consisting of homopolymers ofconjugated dienes and copolymers of conjugated dienes and ethylenicallyunsaturated monomers, the rubber being stabilized with an antiozonantamount of acid-catalyzed, elevated-temperature reaction product of (l)0.5 to 2.0 moles of dialkylol thiourea in which the alkylol groupscontain 1 to 4 carbon atoms, and (2) 1.0 mole ofN,Ndi-sec-butyl-p-phenylenediamine.

2. The vulcanized rubber of claim 1 in which the dialkylol thiourea ofthe reaction product is dimethylol thiourea.

3. Synthetic rubber which is a copolymer of butadiene and styrene whichis sulfur-vulcanized and stabilized with a small amount ofacid-catalyzed, elevated-temperature reaction product of 0.5 to 2.0moles of dimethylol thiourea and 1.0 mole ofN,N'-di-sec-butyl-p-phenylenediamine.

4. The method of vulcanizing natural and synthetic rubbers of the classconsisting of homopolymers of conjugated dienes and copolymers ofconjugated dienes and ethylenically unsaturated monomers, which methodcomprises vulcanizing the same with sulfur in the presence of anantiozonant amount of acid-catalyzed, elevated-temperature reactionproduct of (1) 0.5 to 2.0- moles of dialkylol thiourea in which thealkylol groups contain 1 to 4 carbon atoms, and (2) 1.0 mole ofN,N'-di-secbutyl-p-phenylenediamine.

5. The method of claim 4, using a reaction product as there described inwhich the dialkylol thiourea is dimethylol thiourea.

6. The method of vulcanizing synthetic rubber which is a copolymer ofbutadiene and styrene, which method comprises vulcanizing the same withsulfur in the presence of an antiozonant amount of acid-catalyzed,elevatedtemperature reaction product of 0.5 to 20 moles of dimethylolthiourea and 1.0 mole of N,N'-di-sec-butyl-pphenylenediamine.

Shaw et a1.: Antiozidants for GR-S Rubber, Rubber World, August 1954,vol. 130, No. 5, pp. 637 and 638.

1. SULFUR VULCANIZED RUBBER OF THE CLASS CONSISTING OF NATURAL ANDSYNTHETIC RUBBERS WHICH ARE FROM THE CLASS CONSISTING OF HOMOPOLYMERS OFCONJUGATED DIENES AND COPOLYMERS OF CONJUGATED DIENES AND ETHYLENICALLYUNSATURATED MONOMERS, THE RUBBER BEING STABILIZED WITH AN ANTIOZONANTAMOUNT OF ACID-CATALIZED, ELEVATED-TEMPERATURE REACTION PRODUCT OF (1)0.5 TO 2.0 MOLES OF DIALKYLOL THIOUREA IN WHICH THE ALKYLOL GROUPSCONTAIN 1 TO 4 CARBON ATOMS, AND (2) 1.0 MOLE OFN,N''-DI-SEC-BUTYL-P-PHENYLENEDIAMINE.